Automotive exhaust gas treatment devices have been used as original equipment by automotive manufacturers for many years as a way to reduce the hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions from automotive internal combustion engines.
Some catalytic converters utilize a monolithic structure containing noble metals (e.g., platinum (Pt), palladium (Pd), rhodium (Rh), and/or the like) to provide catalytic afterburning of the engine emissions. More advanced systems utilize a three-way catalytic converter that is capable of simultaneously reducing the emissions of HC, CO. and NOx. To maximize the efficiency of these three-way converters, the engines are typically run at stoichiometry; that is, they are run at an air/fuel ratio in which the amount of air (oxygen) inducted into the cylinder is equivalent to the amount needed to burn all of the injected fuel. The stoichiometric air/fuel ratio is approximately 14.56. One problem with this mode of engine operation is that it is not always possible or desirable to operate the engine at stoichiometry. Rather, for purposes of maximizing fuel economy, it is often desirable to operate the engine in a lean combustion condition in which the amount of intake air is greater than is needed to burn the injected fuel.
Under lean-burn conditions, the air-to-fuel ratio is adjusted to be somewhat greater than the stoichiometric ratio (about 14.7 or greater), generally between about 19 and about 35, in order to realize a fuel economy benefit. However, when operating under lean-burn conditions, typical three-way catalyst systems are efficient in oxidizing unburned HC and CO, but are inefficient in reducing NOx emission. Conversely, during engine warm up and during periods of acceleration when torque is required, it is desirable for driveability to operate the engine in a rich combustion condition in which the amount of fuel injected is greater than the amount of fuel that the inducted air can burn. A rich combustion condition is one in which the air/fuel ratio is less than 14.56.
More recently, NOx adsorbers have been developed which store NOx during periods of lean engine combustion (i.e., excess air) and then periodically release the NOx during periods of rich combustion (i.e., excess fuel) so that the NOx can be catalytically reduced due to the presence of excess HC, CO, and H2.
The amount of NOx that can be stored in a NOx adsorber during any one lean cycle is dependent upon the state, volume, and temperature of the NOx adsorber. A problem with NOx adsorbers is that, over time, the adsorbers can deteriorate due to, for example, poisoning from sulfur oxides (SOx).
When the NOx adsorber has deteriorated to some extent, it needs to be regenerated by running the engine rich until the stored sulfur oxides are released. A common problem that remains unsolved, however, is that during the rich combustion conditions necessary for desulfurization of the NOx adsorber, the amount of HC and CO breakthrough (i.e., released from the system to the environment) increases.
Therefore, a need exists for a more effective system for desulfurization of the NOx adsorber; that is, for an exhaust system that enables improved desulfurization while decreasing HC and CO breakthrough. Additionally, there exists a need for a NOx adsorber diagnostic system that provides an accurate measure of the stored NOx.
There also exists a need for an engine control system, which provides control of the amount of NOx supplied to the NOx adsorber between regenerations.